Part 19

Their plans were: To light the cottage by electricity; introduce a number of electric back logs, with coloured glass bottles; heat the fireless cooker by electricity; pump the water for the house by electricity; run mother's sewing machine by electricity; run the washing machine and wringer by electricity; heat sad irons by electricity; percolate coffee, wash dishes and run the vacuum cleaner by electricity; operate the door bell and the telephone and wind the clock by electricity. I was sure that if they carried out these plans they would stay in Millville at least that summer, so I said go ahead.

We fixed the penstock. The boys estimated that 10 cubic feet of water per second would pass through it. They said that a cubic foot of water weighed 62.5 pounds and 10 cubic feet weighed 625 pounds. They said it fell at the rate of 7 vertical feet a second, making 4375 foot-pounds per second. But 550 foot-pounds per second is one horse-power, hence this is about 8 horse-power. Since one horse-power is equivalent to 746 watts of electricity, we have, if we could generate it without loss, said the boys, nearly the equivalent of 6 kilowatts of electricity, or about 54 amperes at 110 volts.

There were several things they wanted to know before they could go further with their plans.

1. How many of these electrical appliances we would be likely to use at one time.

2. How much current each device would require.

3. How much they must allow for losses in generating the current, in transmitting it, and in transforming it.

We assured them that we would never use more than twenty amperes, say, two thousand watts at one time. They might install a fuse, or circuit breaker in our line to protect their plant against a greater load from us. I told them that they might allow 20 per cent. loss of energy at the dynamo in converting water-power into electric-power.

I would suggest generating their current at 115 e. m. f. and stepping it up to 460 for transmission to us, and then stepping it down to about one hundred and ten volts for our use. They might count on about one-third loss on our supply, that is, they would need to generate about three thousand watts in order to deliver us 2000 watts.

I suggested making our line of No. 6 copper wire, which has a resistance of two ohms to the mile. The distance from the mill to the cottage is one mile, and the complete circuit therefore would require two miles of wire, or four ohms of resistance.

If we start with 3000 watts and lose 14 per cent. in transforming we shall have 2580 watts to transmit. If the voltage has been stepped up fourfold there will be about 5.6 amperes to transmit which will suffer a loss of 22.4 volts in passing through four ohms of resistance on the line. The loss in transmission will be about 5 per cent., and we shall have on arrival at the cottage about two thousand four hundred and fifty watts with a voltage of 437.6. If now in stepping this down to one fourth the voltage, viz., 109.4, we lose 14 per cent., we shall have left something over two thousand one hundred watts, or nearly twenty amperes.

Assuming that you are able to generate 4800 watts of electricity and that 3000 watts must be furnished for transmission to the cottage, you have left 1800 watts, which will give you something over fifteen amperes at 115 volts for use in your machine shop. I suggest that we get a dynamo which will generate both alternating and direct current--the alternating current you will send to the cottage, and the direct current you will have for use at the machine shop.

But how is it possible for a dynamo to generate both alternating and direct current at the same time?

Recall that all dynamos are generators of alternating current. If the brushes rest upon rings upon the axle they send forth alternating current--but if the brushes rest upon commutator bars they send forth direct current. Now we will have two sets of brushes, one pair of which shall rest upon the rings on the axle, and they will collect alternating current for the cottage, while the other pair will slide over the commutator bars and collect direct current for the machine shop. I have constructed a model which will make it plain. Here is a piece of a broom handle (Fig. 138), one foot long, which shall represent the axle of an armature. a b c d is a stout wire which represents the coil of the armature. In this case it has no iron at its centre. Nevertheless it will serve as an armature having one loop of its coil left. e and f are rings, sawed from a piece of brass pipe, which fit snugly upon the axle. Another ring of the brass pipe was sawed lengthwise, as shown in Fig. 139. These two halves are also fastened upon the axle and one end of the wire loop, c, is fastened to one of these, and the other end of the loop, b, is fastened to the other half of the ring. These two halves of the piece of brass pipe are placed so that their edges are near to each other but do not touch on either side of the axle. The two ends of this wire loop are also connected with the rings e and f. A short wire connects b and e and another connects c and f passing through the wood of the axle, as shown by the dotted line. We will now revolve this loop slowly about its axle in a strong magnetic field. To produce this field I will send two amperes of electricity through the coils of wire (Fig. 140), which surround two iron pole pieces that are screwed into an iron base. Between the poles N and S of this electro-magnet we will thrust this wire loop and revolve it as an armature very slowly. Meanwhile I connect two wires to my sensitive ammeter and let their free ends brush along on the rings e and f. The needle of the ammeter swings to and fro for each half revolution of the armature, showing an alternating current of .01 amperes. If this armature had many turns of wire instead of this one loop, if it had an iron core, and if it should revolve at high speed, the results would differ in degree but not in kind.

We will now move the wires which are acting as brushes over to the metal pieces b and c. When now we revolve the armature the needle swings to the right, and just as the needle is about to swing back each brush slides from the plate on which it is rubbing to the opposite one and the needle gets another impulse forward. If the armature is turned rapidly the pulses disappear and the needle stands constantly at about .015 amperes. This then is both an alternating and a direct current dynamo. It simply needs more iron, more copper wire, and more rapid motion, to give us the 4800 watts of electrical energy we are seeking.

"But how shall we produce the current which we wish to send around the spools of the field?" inquired the boys.

"Connect the field with the brushes which rub upon the commutator," I replied. "It will magnetize its own field."

* * * * *

As good luck would have it, we found that the ledge of rock which furnished the basis for the mill dam was immediately underneath the floor at the north end of the machine shop. Upon this we built up a solid foundation for the dynamo. Our water-wheel gave a speed of 240 revolutions per minute to the counter shaft. A pulley of two feet in diameter upon this counter shaft was belted to the pulley of one foot in diameter upon the dynamo--thus giving its armature a speed of 480 revolutions per minute. We had to fix a governor upon the water-wheel to keep this speed constant at varying loads. The voltage is very sensitive to slight changes in the speed of the generator.

We had next to plan what equipment we should need for the machine shop and to decide where to locate each machine. The first machine we installed was a lathe adapted for use both with metals and wood. Among the adjuncts of this were all sorts of drills, chisels, circular saws, grinding and burnishing tools, etc. The second machine located was a small forge with an electric fan to furnish the blast. These were followed by a small band saw and a small planer. The fifth machine was a big grindstone and the sixth was an emery wheel. The boys had a long discussion, running through several days, on the question whether these machines should be belted to the counter shaft, and thus get power directly from the water-wheel, or whether each machine should be operated by an electric motor attached to it.

Harold said: "Suppose I want to saw a piece of wood requiring a horse-power, I must start an eight horse-power water-wheel which will run a six-horse-power dynamo which will operate a two-horse-power motor that will revolve the saw. There is a loss in each machine, and the lighter the load the greater the loss. In order that the motor may deliver one horse-power to the saw, it must receive from the dynamo, say, one and one-half horse-power, and in order that the dynamo may deliver to the motor one and one-half horse-power, it must receive from the water-wheel, say, two horse-power. What is the matter with my saving time and energy by sawing off the block with my own right arm?"

"But," said Ernest, "you forget that this water-wheel and the dynamo must run all the time by the terms of our agreement with the cottage, and they will run fairly well loaded, so that the starting of the saw will not entail any such losses as you reckon. Furthermore the water-power is running to waste, anyway. You simply divert its channel when you start all this machinery. That's all. And lastly, if the saw requires a horse-power, as you say, your right arm could not furnish it."

"Oh," interposed Dyne, "it would take a horse-power to do it as quickly as the machine does, but Harold simply proposes to take more time in sawing the block and less in running the machinery. An infant can do the work of a horse if you give him proportionally more time."

"I don't like the idea," drawled Erg, "that this machinery has got to be kept running all the time. When will a fellow get a chance to sleep or go a-fishing or have any vacation, with this central-station machine shop on his hands all the time?"

I had inquired how the last two boys won their nicknames of Dyne and Erg and had been informed that one was very keen about dining and the other had a great aversion for work. They had doubtless seen these terms somewhere in their reading of physics, but they appeared to have forgotten their significance by a too familiar use of them. I told them that these were sacred terms, the first being a name for the unit of force, while the second designated the unit of work. Both boys said that under those circumstances they would like to shed the names. The names, however, stuck and the boys themselves might, I think, be said to exercise a maximum of power with the least waste of energy.

This idea of running the plant continuously had evidently received no attention hitherto and it bid fair to quench all the enthusiasm until I came to the rescue by proposing a storage battery.

If we can procure a battery in which we may store energy, which shall always be on draught by merely pushing a button, one which "is not injured by overcharging nor too rapid discharging, nor even by complete discharge"; one which is not injured by standing idle for any length of time, either charged or discharged; and finally one which "is practically foolproof"--we want to try it. I propose that you appoint a committee to look into it. But at any rate this enterprise must go on even if I have to hire a man to live in the loft of the mill and keep the machinery going.

"No man in the loft," said Dyne, "while I have my rations."

"There will be no need for him so long as I can store energy here," said Erg, "so let the job of equipping the establishment go on in the regular fashion."